This study is to define ethnic differences in post-synaptic mechanisms of vascular response to adrenergic agonists, and in particular to localize the sites of previously identified attenuation of vasodilation to isoproterenol in African Americans. Also, the effects of ethnicity on central and peripheral mechanisms of adrenergic control and to define the sites of such differences and their regulation will be studied. This study has been expanded to include the effects of estrogen on vascular reactivity.